1. Field of the Invention
The present invention relates to a process for producing a plastic lens. More particularly, the present invention relates to a process for producing a plastic lens which prevents formation of striaes in a lens since it is suppressed that the material monomer composition in an excess amount and present in the inlet of a plastic gasket is drawn into the cavity of a mold for producing a lens during the polymerization, enables to obtain a plastic lens having a uniform composition since the material monomer composition is not affected by rapid and excessive heating and enables to perform the operations for producing the plastic lens efficiently at a high speed.
The present invention also provides a process for producing a plastic lens which provides a plastic lens exhibiting excellent transparency without contamination with foreign substances or clouding and enables to perform the operations for producing the plastic lens efficiently at a high speed since the possibility of contamination of the inside of the cavity due to scattering of powder of the gasket material formed at the portion of the gasket treated by the ultrasonic welding can be eliminated in the ultrasonic welding of the inlet of the plastic gasket.
2. Description of the Related Arts
Recently, plastic lenses have been widely used due to the advantages in that the weight is lighter and the safety is greater than those of inorganic glass lenses. In particular, the diethylene glycol bisallyl carbonate resin (referred to as “CR-39 resin”, hereinafter) has been mainly used for the spectacle lenses. However, this resin has a problem in that the refractive index is as small as about 1.50, and the thickness of a lens using the resin is greater than that of glass lenses. Therefore, various proposals have been made to increase the refractive index of the plastic lens (for example, Patent References 1 and 2). Polythiourethane resins obtained by the reaction of a polyisocyanate compound and a polythiol compound which are described in Patent Reference 1 and plastic lenses obtained by polymerization of a compound having epithio group, a polythiol compound and a polyisocyanate compound which are described in Patent Reference 2 are being widely used due to the high refractive index and the great Abbe constant.
In general, the process for producing a plastic lens comprises a step of material preparation in which the main monomer used as the material of the lens and necessary additives are weighed, mixed and stirred to prepare a solution having a uniform composition, a step of injection in which the prepared solution is injected into a mold for producing a lens comprising mold members made of glass or a metal and a gasket made of a plastic, a step of polymerization in which the polymerization is conducted in accordance with a suitable temperature program and a step of separation from the mold in which the lens obtained by the polymerization is taken out of the mold.
When the mold for producing a lens is a mold of the closed type which is constituted with mold members and a gasket having no inlet, the sealing portion of the mold is forced to open in the step of injection, a nozzle is inserted into the formed gap, and a monomer composition is injected into the cavity of the mold through the nozzle. To surely fill the cavity with the monomer composition without leaving bubbles at the inside of the cavity, the monomer composition in an excess amount is injected into the cavity, and the mold is closed by closing the gap at the sealing portion while the monomer composition in the excess amount is expelled to the outside.
In this case, the monomer composition expelled to the outside fouls outside portions of the mold and apparatuses for the production such as a rack used in the production, and this causes drawbacks in that an apparatus for cleaning the fouling is necessary and the lens obtained as the product is fouled. Therefore, the yield of the production is decreased, and the production of the lens may be made impossible depending on the properties of the monomer composition.
Recently, a process in which an inlet used for the monomer composition alone is formed at the gasket and the monomer composition is injected through the formed inlet is becoming popular. In accordance with this process, in general, the monomer composition is injected into the cavity of the mold to gradually fill the cavity. The injection is stopped when the liquid surface reaches the inlet, and the mold is closed by sealing the inlet. In accordance with this process, there is no possibility that the monomer composition overflows to the outside of the mold to cause the above problems. As the process for sealing the inlet, as disclosed in Patent Reference 3, a process in which heat sealing is conducted using a film obtained by laminating a thermoplastic elastomer to an aluminum foil or a plastic film is known.
However, when the rapid and excessive heating in the heat sealing affects the monomer composition, abnormal reactions tend to take place, and it becomes difficult that a uniform plastic lens is obtained. Therefore, it is necessary that the portion for the heat sealing be separated from the monomer composition by some distance so that the heating does not affect the monomer composition. Since the monomer composition in the portion of the inlet is drawn toward the cavity due to contraction of the volume in the step of polymerization, it is necessary that the monomer composition at least in the amount corresponding to the amount drawn toward the cavity be present in the portion of the inlet. When it is difficult that the amount of the liquid present in the portion of the inlet is controlled accurately in the injection, it is necessary that the amount of the liquid present in the portion of the inlet be increased so that the difficulty caused by the fluctuation in the amount is overcome. When the amount of the monomer composition drawn toward the cavity from the portion of the inlet exceeds a specific value, the effect of the monomer composition drawn toward the cavity on the polymerization cannot be neglected, and the condition of the polymerization in this region becomes uneven. Due to this effect, characteristic striaes are frequently formed in the produced plastic lens at the portions in the vicinity of the inlet. A process which can overcome the above problems has been desired.
[Patent Reference 1] Japanese Patent Application Laid-Open No. Heisei 7(1995)-316250.
[Patent Reference 2] Japanese Patent Application Laid-Open No. 2001-330701
[Patent Reference 3] Japanese Patent Application Laid-Open No. Heisei 7(1995)-164550.